A. Field of The Invention
The invention relates to scanners for providing a digitized image of a document and, more particularly, to a focusing system for automatically focusing the optical head of a drum scanner.
B. Prior Art
Rotary scanners mount an image source such as a document or film on a drum for scanning by an optical scanning head. Relative rotational and translational motion between the drum and the head is provided to form successive scan lines.
Rotary scanners typically provide either a fixed focus system or a manually adjustable focusing system. The former is the least expensive to implement, but assumes documents which are of relatively uniform thickness, since this can significantly affect the focus of the system. A manually adjustable system allows adjustment of the focus but often requires operators who have the skill to select a focus that will provide acceptable results for the document being scanned. This results in part because the optical head typically has a large numerical aperture for efficient collection of light but accordingly also has a shallow depth of field.
Skilled operators are commonly employed in connection with rotary drum scanners, since such scanners are frequently comparatively quite expensive and are commonly used in high volume, commercial production systems. However, the necessity for skilled operators is a drawback in connection with "low end" or moderately priced rotary drum scanners (e.g., under $10,000 at present prices), since such scanners are increasingly used by office personnel with limited or no training in scanner operation.